The confectionery giant is set to remove artificial colours, flavourings and preservatives from Mars, Twix, Milky Way and Snickers bars.
Chocolate experts insist the alterations will not change the taste but will improve the waistline.
The move is part of Mars' Raising The Bar campaign that aims to improve the health of its customers.
After undergoing the makeover, funsize versions of Mars will contain fewer than 100 calories and nutritional information will be clearly displayed on products.
"We're constantly looking for ways to make the products better for the consumer," The Daily Star quoted a company spokesman, as saying.
"However, we're still a treat food and should be seen as a small bit of pleasure in your day," the rep added.